This project will deal with a physicochemical investigation and comparison of the properties of the cytoplasmic and mitochondrial forms of the enzyme malate dehydrogenase isolated from porcine heart. Recent work by this laboratory has implicated the essentiality of four residues in the active center of the mitochondrial enzyme, i.e. (histidine, cysteine, lysine and arginine). In addition, investigative work by this laboratory has implicated the essentiality of an arginine residue in the active center of the cytoplasmic form of the enzyme. Sequence studies are being pursued on labeled peptides containing these residues in order to identify other residues in their proximity. Bifunctional reagents will be utilized in order to investigate the proximity of each of these active center residues to one another. The objective of this work will be to attempt to understand the function of each of these residues in the mechanism of action of these enzymes.